Usually an articulated arm coordinate measuring machine (short articulated arm CMM) is built by a heavy base for a stable standing and an articulated arm connected to the base. The articulated arm comprises various arm sections connected by pivot joints, wherein the final distal arm section is provided with a tool for measuring a part-to-be-measured, i.e. a tactile or optic measurement probe, or with a tool holder allowing exchange of the tool in order to adapt the tool to various measurement requirements. Examples for such articulated arm CMMs can be found i.e. in EP 2108917 A1 or EP 2283311 B1. The tool or measurement probe can be a tactile probe, like a ruby ball or an optical probe; both types of probe are well known in various embodiments by a skilled person. The holder can be equipped with buttons for controlling the measurement and can be arranged by itself in a rotatable or fixed way.
For measurement results of high precision it is necessary to know exactly the position of the measuring probe at the distal end of the arm of the articulated arm CMM in relation to a reference point, which reference point might be given by a particular point of the base of the articulated arm CMM or with respect to the part-to-be-measured. For this purpose angle encoders are placed at each pivot joint and in case of telescopic arm sections linear position encoders are provided. The encoders deliver their encoder signals to a central circuit, which central circuit is configured to derive the position of the arms relative to each other and/or relative to a reference point or reference coordinate system. The reference coordinate system is often created in relation to the part-to-be-measured by carrying out a kind of calibration measurement before starting the detailed measurement of the part-to-be-measured. In other words, the measurement process starts taking some reference measurements on the part-to-be-measured itself, for example: reference plane, reference edge and reference point. Thus, six degrees of freedom including the orientation of the probe relative to the part-to-be-measured are defined before the beginning of the detailed measurement. Subsequently all measurements are given relative to this reference.
Usually an articulated arm coordinate measuring machine is built by an external frame encapsulating susceptible components like the encoders, equipment for power supply and/or electronic signal transmission and so on. An example for such an articulated arm CMM is given in DE 4403901 A1. Although single components of this articulated arm CMM are modular in order to allow variable assembly configurations, the articulated arm CMM as such has to be constructed beforehand as a whole; subsequent amendments in order to adapt the articulated arm CMM to the requirements of the user is hardly possible and can only be realized with a considerable effort in work and time, because of the huge amount of single elements assembled within such an articulated arm CMM. In order to reduce the amount of single elements DE 60318396 T2 proposes to integrate the angle encoder into the pivot joint. However the amount of single elements is still considerable and causes a relative high weight even for transportable articulated arm CMMs as discussed in DE 4403901 A1 and DE 60318396 T2.